In principle, the known machines of this type comprise a stator having a cylindrical internal part of radius Rs and a rotor of generally cylindrical shape of radius Rr smaller than Rs. The rotor, which is eccentric with respect to the stator, is tangent internally to the stator. Fluid-tightness between the admission and the delivery is therefore produced only by a tangent line, which does not provide ideal fluid-tightness.
In some cases, a static scraper pressing against the rotor has been tried, but this device dissipates energy and does not achieve an appreciable improvement in the fluid-tightness.
Indeed, in order to achieve fluid-tightness, it would be necessary for said scraper to be machined across its total width to the diameter of the rotor. Now this is impossible because the blades would come and strike against it. Consequently the scraper must be machined to the diameter of the stator to permit the passage of the blades, which brings us back to a tangent line and does not solve the fluid-tightness problem.